Surface-treated article of magnesium or magnesium alloys, method of surface preparation and method of coating

ABSTRACT

An article of magnesium or its alloy having a surface layer containing magnesium condensed phosphate and magnesium phosphate, which can be produced by treating an article of magnesium or its alloy with a treating liquid containing 1,000 to 20,000 ppm of alkaline metal ions, 1,000 to 50,000 ppm of condensed phosphate ions, and 100 to 20,000 ppm of borate ions, and having pH of at least 8. The treated article has good corrosion resistance, and a coating can be formed on the surface of the article with good adhesion.

FIELD OF THE INVENTION

The present invention relates to a surface-treated article of magnesiumor magnesium alloys, a method of the surface preparation of an articleof magnesium or magnesium alloys, and a method of the coating of such anarticle. In particular, the present invention relates to a method of thesurface preparation of the article of magnesium or magnesium alloys(hereinafter referred to as “magnesium article”) which can perform thedegreasing and chemical conversion treatment of the magnesium article inone step, and a method of the coating of a magnesium article, which isperformed in combination with the above surface preparation.

PRIOR ART

Housings of CRTs used in home appliances such as TVs, word processors,personal computers, etc. are made of plastics from the viewpoint ofstrength, and easiness of production. However, with the rise of concernabout environmental quality, there arise some trends towards thereplacement of plastics with materials which can be easily recycled.

In the case of plasma displays which are vigorously developed asdisplays, it is desired to produce their housings with materials havinghigh electromagnetic shielding effects.

Thus, it may be contemplated to produce housings with metals. Metalhousings should satisfy various requirements, for example, light weight,sufficient strength, causing no environmental problems, good corrosionresistance, etc. Metals which satisfy such requirements includemagnesium and its alloys.

When magnesium and its alloys are used as the materials of housings, thehousings are coated to improve their appearances. However, it isnecessary to subject the housings to surface preparation (pre-treatment)to improve the adhesion of coatings and corrosion resistance.

Hitherto, the surface preparation of magnesium articles has been carriedout by a JIS method or a DOW method, which uses chromium compounds.Since the chromium compounds are toxic compounds, a waste watercontaining chromium compounds cannot be drained into sewers withoutsophisticated post-treatment.

JP-A-6-330341 discloses, as a surface preparation method of articles ofmagnesium or its alloys using no chromium compounds, a method using azinc phosphate-treatment liquid containing zinc ions, manganese ions,phosphate ions, a fluorine-containing compound and an accelerator forthe chemical conversion of a film.

However, this method is not satisfactorily harmless to the environmentsince the treatment liquid contains heavy metals such as zinc andmanganese, and also fluoride ions which make it difficult to treat awaste water.

JP-A-6-116740 discloses a surface-treating method comprising treating anarticle of a magnesium alloy containing aluminum with a pyrophosphatesalt solution and then with the solution of an alkali metal hydroxide.

This method is good since it uses neither toxic materials nor heavymetals, but is not an economically advantageous method since itcomprises a number of process steps.

JP-A-5-51679 discloses a method for the formation of a protectivecoating by anodization comprising the steps of dipping the article ofmagnesium or its alloy in a low-alkalinity aqueous electrolytic solutioncontaining borate or sulfate anions, phosphoric acid and fluoride orchloride ions and depositing magnesium phosphate and magnesium fluorideor chloride on the surface of the article of magnesium or its alloywhile applying a direct current through the article.

This method is not satisfactorily harmless to the environment since itcontains fluoride ions which make it difficult to treat a waste water.Furthermore, this method is not advantageous from the viewpoint of costssince it requires apparatuses such as a rectifier, electrodes, etc. toapply a direct current.

The surfaces of magnesium articles, which are produced by die casting orthixomolding, are contaminated with releasing agents, oxide films,greases, etc. Thus, it is necessary to remove such contaminants toadjust the surface conditions. To this end, the surfaces are degreased,pickled, or subjected to the surface adjustment, prior to chemicalconversion.

These treating methods have drawbacks such that the number of treatingsteps is large, the number of items for controlling liquids is large,the amount of waste water is large, and so on. Furthermore, the chemicalconversion liquids contain many materials which are controlled from theviewpoint of environmental protections, and thus the load on waste waterdisposal and influences on the environment are concerned about.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a surface-treatedmagnesium article having good corrosion resistance, on which coatingscan be formed with good adhesion, Another object of the presentinvention is to provide a method for the surface preparation of amagnesium article, and also a method for the coating of a magnesiumarticle, which can reduce the number of process steps, improve thecorrosion resistance of the magnesium article, and have less influenceson the environment.

The above and other objects can be accomplished by

(1) an article of magnesium or its alloy having a surface layer whichcontains magnesium condensed phosphate and magnesium phosphate,

(2) a method for the surface preparation of an article of magnesium orits alloy comprising treating the article with a treating liquid whichcontains

1,000 to 20,000 ppm of alkaline metal ions,

1,000 to 50,000 ppm of condensed phosphate ions, and

100 to 20,000 ppm of borate ions,

and has pH of at least 8, and

(3) a method for coating an article of magnesium or its alloy comprisingthe steps of:

treating the article with a treating liquid which contains

1,000 to 20,000 ppm of alkaline metal ions,

1,000 to 50,000 ppm of condensed phosphate ions, and

100 to 20,000 ppm of borate ions,

and has pH of at least 8,

removing the article from the treating liquid,

washing the article with water,

drying the washed article, and

coating the dried article.

DETAILED DESCRIPTION OF THE INVENTION

Articles to be treated by the method of the present invention are thosemade of magnesium or its alloys. In particular, such articles arehousings of CRT displays for TVs, word processors, personal computers,but are not limited to them.

Such articles can be produced by any conventional methods, for example,sheeting, die casting, and the like.

The compositions of magnesium alloys are not limited, and any magnesiumalloys, which are industrially used, may be used in the presentinvention.

Preferred examples of magnesium alloys include magnesium alloyscontaining aluminum (e.g. AZ-91A, B and D, etc.).

Examples of condensed phosphate salts, which are contained in a treatingliquid used in the method of the present invention, include salts ofpolyphosphoric acid, metaphosphoric acid, and ultraphosphoric acid.Among them, polyphosphoric acid of the formula: M_(n+2)P_(n)O_(3n+1) inwhich n=2, 3, 4, . . . , in particular, polyphosphoric of this formulain which n is 3 is preferable.

The dispersibility of polyphosphate salts in water decreases as themolecular weight of polyphosphoric acid increases. hen the molecularweight of polyphosphoric acid is too low, the unctions of condensedphosphate which the present invention uses, for example, sequesteringproperties, buffering properties, dispersing properties, etc. maydeteriorate.

Condensed phosphoric acid tends to be hydrolyzed in an aqueous medium ina neutral or acidic range to form orthophosphoric acid throughtripolyphosphoric acid and pyrophosphoric acid. The rate of hydrolysisdepends on pH and a temperature. That is, the rate of hydrolysisincreases as pH decreases or as a temperature increases. Thus, atreating liquid used in the present invention has pH of at least 8,preferably in the range between 8 and 11, and a treating temperature ispreferably 70° C. or less.

Magnesium articles can be mildly etched when the sequestering propertiesare used among. the various properties of condensed phosphoric acid. Thesurfaces of magnesium articles can be cleaned by etching with condensedphosphoric acid, and cleaning effects of optionally used surfactants.The etched amount of magnesium articles increases, as the concentrationof condensed phosphate salts increases and a treating time is extended.

In the case of magnesium alloys, if the surface of magnesium alloyarticles are excessively etched, smuts, that is, oxides and hydroxidesof aluminum, zinc, etc., cover the surfaces of magnesium articles, andthus the adhesion of coatings to the surface may deteriorate.

If the etched amounts of magnesium articles are too large, the sizes ofmagnesium articles excessively change so that the method of the presentinvention cannot be adapted to precise parts of today. When the etchedamounts of magnesium articles are too low, the surfaces of magnesiumarticles may not be sufficiently cleaned.

An optimum etched amount is from 0.1 to 20 g/m², preferably from 0.5 to10 g/m². An etched amount is not limited in the above range, since itdepends on the surface conditions of magnesium articles.

As described above, condensed phosphoric acid is hydrolyzed in anaqueous medium to form tripolyphosphoric acid and pyrophosphoric acidand finally orthophosphoric acid. A part of pyrophosphoric acid andorthophosphoric acid, which are hydrolyzed products, react withmagnesium to form the film of magnesium condensed phosphate andmagnesium phosphate. This chemically formed film improves the corrosionresistance of magnesium articles and the adhesion of coatings tomagnesium articles.

The amount of such a film is at least 5 mg/m², preferably in the rangebetween 10 and 100 mg/m² in terms of P (phosphorus).

The concentration of condensed phosphate ions is in the range between1,000 and 50,000 ppm, preferably in the range between 3,000 and 30,000ppm, as described above. When the concentration of condensed phosphateions is less than 1,000 ppm, the etching effect is low and a treatingliquid tends to be easily aged. When the concentration of condensedphosphate ions exceeds 50,000 ppm, an etched amount becomes excessive,and the adhesion of coatings tends to decrease since smuts (oxides andhydroxides of aluminum, zinc, etc.) cover the surfaces of magnesiumarticles. Furthermore, the too high concentration of condensed phosphateions is disadvantageous from the viewpoint of costs.

According to the present invention, a treating liquid contains borateions, which have buffering properties, to improve the corrosionresistance of magnesium articles and to suppress the hydrolysis ofcondensed phosphate ions.

The concentration of borate ions is in the range between 100 and 20,000ppm, preferably in the range between 500 and 10,000 ppm. When theconcentration of borate ions is less than 100 ppm, a sufficientbuffering effect is not attained. When the concentration of borate ionsexceeds 20,000 ppm, the effects may not be further improved, and thussuch an excessive concentration is disadvantageous from the viewpoint ofcosts.

Alkali metal ions are counter ions to condensed phosphate ions andborate ions, and include sodium ions, potassium ions, etc. Among them,sodium ions are preferable in the present invention.

The concentration of alkali metal ions depends on the concentrations ofphosphate ions and borate ions, pH, etc., and is generally in the rangebetween 1,000 and 20,000 ppm, preferably in the range between 5,000 and15,000 ppm.

When the concentration of alkali metal ions is less than 1,000 ppm, pHcannot be maintained in a suitable range and thus the stability ofcondensed phosphate ions may be adversely affected. When theconcentration of alkali metal ions exceeds 20,000 ppm, the effects maynot be further improved, and thus such an excessive concentration isdisadvantageous from the viewpoint of costs.

A treating liquid used in the present invention may contain surfactants,chelating agents, defoaming agents, etc., which are contained inconventional degreasing agents, in a suitable concentration, so as toclean and remove contaminants such as greases or releasing agents whichadhere to magnesium articles, sequester hard water components, suppressfoaming, and so on.

Surfactants are useful as cleaning-improving agents against greases,etc. and contribute to penetration, emulsifying and dispersingproperties.

Surfactants to be used in the present invention include (a) nonionicsurfactants, (b) anionic surfactants, (c) cationic surfactants, and (d)ampholytic surfactants.

The concentration of surfactants is not limited since it depends on thesurface conditions of magnesium articles, treating methods, etc. Such aconcentration is usually in the range between 10 and 2,000 ppm,preferably in the range between 500 and 1,000 ppm.

When the concentration of surfactants is less than 10 ppm,emulsification and dispersibility of greases may decrease, and theadhesion of coatings to the articles may deteriorate. When theconcentration of surfactants exceeds 2,000 ppm, the effects may not befurther improved, and thus such an excessive concentration isdisadvantageous from the viewpoint of costs.

Preferred but non-limiting examples of surfactants are as follows:

(a) Examples of nonionic surfactants include polyoxyethylene alkylethers (having 6 to 16 carbon atoms in the alkyl group),polyoxyethylenepolyoxypropylene alkyl ethers (having 6 to 16 carbonatoms in the alkyl group), polyoxyethylene alkylaryl ethers (having 6 to16 carbon atoms in the alkyl group, and the aryl group being usually aphenyl group), polyoxyethylenepolyoxypropylene alkylaryl ethers (having6 to 16 carbon atoms in the alkyl group, and the aryl group beingusually a phenyl group), polyoxyethylene alkylaminoethers (having 6 to16 carbon atoms in the alkyl group), polyoxyethylenesorbitan higherfatty acid esters (usually monoesters; the higher fatty acid being asaturated or unsaturated C₁₂-C₁₈ monobasic fatty acid), polyoxyethylenehigher fatty acid esters (mono- or diesters; the higher fatty acid beinga saturated or unsaturated C₁₂-C₁₈ monobasic fatty acid), ethyleneoxide-propylene oxide copolymers, and the like.

Examples of nonionic surfactants which are particularly effective forthe cleaning purpose are

polyoxyethylene hexyl ethers,

polyoxyethylene octyl ethers,

polyoxyethylene decyl ethers,

polyoxyethylene lauryl ethers,

polyoxyethylene octylpheyl ethers,

polyoxyethylene nonylphenyl ethers,

polyoxyethylene decylphenyl ethers,

ethylene oxide-propylene oxide copolymers,

polyoxyethylenepolyoxypropylene nonylphenyl ethers, etc.

The nonionic surfactants may be used independently or in combination oftwo or more.

(b) Examples of anionic surfactants include higher fatty acid salts (thehigher fatty acid usually being a saturated or unsaturated C₁₂-C₁₈monobasic fatty acid), alkylsulfate ester salts (having 12 to 18 carbonatoms in the alkyl group), alkylbenzenesulfonate salts (having 11 to 15carbon atoms in the alkyl group), alkylnaphthalenesulfonate salts(having about 4 carbon atoms in the alkyl group), dialkylsulfosuccinateester salts (having 10 to 20 carbon atoms in total in two alkyl groups),alkylphosphate ester salts, (having 12 to 18 carbon atoms in the alkylgroup), formalin condensed naphthalenesulfonate salts, polyoxyethylenealkylsufonate ester salts (having 8 or 9 carbon atoms in the alkylgroup), polyoxyethylene alkylphenylsulfonate ester salts (having 8 or 9carbon atoms in the alkyl group), etc.

The above salt-form anionic surfactants are usually alkali metal salts.Among them, sodium salts are preferable.

The anionic surfactants may be used independently or in combination oftwo or more.

(c) Examples of cationic surfactants include alkylamine acetates,alkylamine hydrochloride salts, quaternary ammonium salts, etc.

(d) Examples of ampholytic surfactants include alkyldimethylamineoxides, alkylcarboxymethylhydroxyethyl-immidazoriumbetaines,alkylaminocarboxylate salts, etc.

The cationic or ampholytic surfactants may be used independently or incombination of two or more.

The treating liquid to be used in the present invention may additionallycontain chelating agents to improve the cleaning effects of the treatingliquid.

The concentration of chelating agents is not limited, since it dependson the hardness of water to be used. The concentration of chelatingagents is usually in the range between 10 and 2,000 ppm, preferably inthe range between 500 and 1,000 ppm. When the concentration of chelatingagents exceeds 2,000 ppm, the effects may not be further improved, andthus such an excessive concentration is disadvantageous from theviewpoint of costs.

Preferred but non-limiting examples of chelating agents includeaminocarboxylic acids (e.g. nitrilotriacetic acid,ethylenediaminetetraacetic acid, ethylenediaminediacetic acid,triethylenetetramine-hexaacetic acid, etc.) and their salts such asalkali metal salts (e.g. sodium salts, potassium salts, etc.), ammoniumsalts and lower alkylamine salts (e.g. triethylamine salts); andhydroxycarboxylic acids (e.g. malic acid, citric acid, gluconic acid,heptogluconic acid, etc.) and their salts such as alkali metal salts(e.g. sodium salts, potassium salts, etc.) and lower alkylamine salts(e.g. triethylamine salts).

Among them, hydroxycarboxylic acids and their salts, in particular,alkali metal salts are preferable.

Chelating agents maybe used independently or in combination of two ormore.

Defoaming agents are added to suppress foaming in a cleaning process.The concentration of defoaming agents is not limited since it depends ona treating method such as spraying, dipping, etc. The concentration ofdefoaming agents is usually in the range between 10 and 2,000 ppm,preferably in the range between 500 and 1,000 ppm. When theconcentration of defoaming agents exceeds 2,000 ppm, the effects may notbe further improved, and thus such an excessive concentration isdisadvantageous from the viewpoint of costs.

Preferably, defoaming agents have low solubility in water and can belightly dispersed in water, or separated from water. Examples of suchdefoaming agents include sorbitan higher fatty acid monoesters (thehigher fatty acid being a saturated or unsaturated C₁₂-C₁₈ monobasicfatty acid), sorbitan higher fatty acid triesters (the higher fatty acidbeing a saturated or unsaturated C₁₂-C₁₈ monobasic fatty acid), adductsof nonylphenol and a few moles of ethylene oxide, ethyleneoxide-propylene oxide copolymers having the small number of addedethylene oxide molecules, polyoxyethylenepolyoxypropylene polyhydricalcohol ethers having the small number of added ethylene oxidemolecules, etc.

Defoaming agents maybe used independently or in combination of two ormore.

The pretreatment of the surface of a magnesium article with a treatingliquid may be carried out by allowing the treating liquid in contactwith the surface. For example, the pretreatment can be carried out bydipping an article in a treating liquid, spraying a treating liquid onan article, or the combination of dipping and spraying. In addition, anymethods which can allow a treating liquid in contact with the surface ofan article may be used.

A temperature in surface preparation is usually in the range betweenroom temperature (about 20 to 25° C.) and 80° C., preferably in therange between 40 and 70° C.

In the case of spraying, a spraying time is usually in the range between1 and 60 minutes, preferably in the range between 2 and 15 minutes.

In the case of dipping, an article is dipped in a treating liquid sothat the sufficient amount of the liquid adheres to the surface of thearticle for a sufficient time for treating the surface. Such a dippingtime can be selected by a persons skilled in the art.

After the surface preparation, a magnesium article is washed with waterpreferably twice or more, although the article may be washed once.

A temperature and a time for washing are not limited. The temperature isnot so high and a short washing time is preferable, since magnesiumarticles react with water even in a neutral water to form brittlemagnesium hydroxide. In principle, the washing of pretreated magnesiumarticles with water may be carried out by substantially the same methodas that carried out after chemical conversion in the conventionalsurface preparation.

After washing with water, water is drained off from magnesium articles,and then the articles are dried, like in the conventional surfacepreparation.

The surfaces of magnesium articles, which have been subjected to theabove surface preparation, carry a layer comprising magnesium condensedphosphate and magnesium phosphate on their surfaces.

After the surface preparation according to the present invention,another chemical conversion treatment may be carried out to furtherimprove corrosion resistance. Such a chemical conversion treatment maybe carried out using sodium phosphate, etc., like the conventionalmethods.

After the surface preparation and optional chemical conversiontreatment, magnesium articles are coated.

A coating method is not limited, and any of conventional coatings suchas solution type coatings, aqueous coatings, powder coatings, etc. maybe used. Electrodeposition coating or powder coating is preferable fromthe viewpoint of environmental protection. In particular, powder coatingis preferable.

It is possible to coat all or a part of the surfaces of a magnesiumarticle.

For example, in the case of the housings of electronic or electricappliances, only the external surface of the housing is coated, whilethe internal surface may remain uncoated. In the case of the housings ofTVs, a coating is sprayed from the side of an opening for installing aCRT screen so that the external and internal surfaces are coated at thesame time.

The degreasing and chemical conversion of magnesium articles can becarried out in one step when the treating liquid of the presentinvention is used. Thus, it is possible to avoid the washing of articleswith water, which is carried out between a degreasing step and achemical conversion step in conventional methods. Accordingly, it ispossible to greatly decrease a total coating time and total costsincluding surface preparation, and the amount of water used for washingcan be decreased. Since the amount water used for washing is decreased,a load on waste water disposal can be decreased. Therefore, the presentinvention is friendly to environments.

EXAMPLES

The present invention will be illustrated by the following examples.

Examples 1-6 and Comparative Examples 1-6

A housing for TV was produced by molding a magnesium material (AZ 91D)and cutting gates.

In Examples 1-6 and Comparative Examples 1-4, an aqueous liquidcontaining the components in concentrations shown in Table 1 was sprayedon the housing at 60° C. for 3 minutes.

The details of the treating method in Comparative Example 5 were asfollows:

A test piece was degreased with trichloroethylene, dipped fin a 3%aqueous solution of hydrogen fluoride for one minute, washed with water,and then dipped in the Dow No. 7 method solution (containing 150 g/l ofdichromic acid and 2.5 g/l of magnesium fluoride) at a temperature of95° C. or higher for 30 minutes.

The details of the treating method in Comparative Example 6 were asfollows:

A test piece was degreased with trichloroethylene, dipped 15 in thesolution of sodium pyrophosphate (30 g/l) at 30° C. for 3 minutes,washed with water, and then dipped in an aqueous solution of sodiumhydroxide at 30° C. for 3 minutes.

The housing, which had been treated as described above, was showerrinsed with water at room temperature for 3 minutes, and then washedwith pure water at room temperature. Then, the housing was dried withhot air at 200° C. for 10 minutes.

Subsequently, a powder coating containing a rust-preventive and a curingaccelerator (“HAMMERTON SILVER” manufactured by OHASHI ChemicalIndustries, Ltd.; an epoxy rein-polyester resin base coating) was powdercoated on the housing, and baked and dried at 200° C. for 10 minutes.Thus, a coating film having a thickness of about 80 μm was formed.

The chemically converted surface and coated surface of the housing weresubjected to the following measurements and tests:

1) Etched amount

A difference between the weight of a housing prior to the treatment andthat after the treatment was defined as an etched amount.

2) Deposited amount of phosphorus

The deposited amount of phosphorus in the formed layer was measured by aX-ray fluorescent analysis.

3) Amount of residual carbon

The amount of residual carbon was measured to evaluate the residue of areleasing agent.

Apparatus: TOC-500A manufactured by SHIMADZU Corporation

Furnace temperature: 500° C.

4) Initial adhesion

A cross-cut test was performed according to JIS K 5400, and the initialadhesion of a coated film was evaluated by the number of remainedcross-cut pieces. The percentage of the remaining pieces of 90% or morewas regarded as “PASS”.

5) Salt spray test

According to JIS Z 2371, a coated film was cut to reach a base metal, a5% saline was sprayed on the coated film for 8 hours and the sprayingwas paused for 16 hours, at 35° C., in a one cycle. This spraying andpausing were repeated three cycles. Then, the condition of the coatedfilm were evaluated according to the following criteria:

A: Neither corrosion nor detachment of the coating film were found.

B: Corrosions of less than 2 mm and detachment of the coating film werefound.

C: Corrosion of 2 mm or larger an detachment of the coating film werefound.

The results are summarized in Table 2.

In Examples 1-6, which used the surface preparation method of thepresent invention, the deposited amount of phosphorus was 10 mg/m² orlarger, the amount of residual carbon was much less than 100 mg/m², andthe initial adhesion of the coated films and the results of the saltspray test were good.

Results of Comparative Examples 1-6 Comparative Example 1

The etched amount was small and thus the surface cleaning properties onmagnesium articles were not good, since the concentration of condensedphosphate ions was low. Furthermore, both the initial adhesion of thecoating film and the result of the salt spray test were bad, since theamount of the layer formed by the chemical conversion was small.

Comparative Example 2

The etched amount was large since the concentration of condensedphosphate ions was high. Thus, smuts covered the surface of themagnesium article and had the adverse effects on the initial adhesion ofthe coating film and also the result of the salt spray test.

Comparative Example 3

The etched amount was not large and thus the surface cleaning propertieson magnesium articles were not good, since the treating liquid containedno condensed phosphate ions but orthophosphate ions. Furthermore, theinitial adhesion of the coating film and the result of the salt spraytest were not good, since the amount of the chemically converted layerwas small.

Comparative Example 4

The etched amount was small, the surface cleaning properties on themagnesium surface were not good, and also the initial adhesion of thecoated film and the results of the salt spray test were bad, sincecondensed phosphoric acid was hydrolyzed because of the low pH of thetreating liquid.

Comparative Example 5

The initial adhesion of the coated film and the result of the salt spraytest were good since the Dow No. 7 method was used. However, thetreating liquid contained fluoride ions, chromium ions, etc., and thenumber of the process steps was large. Thus, this method wasinappropriate from the viewpoint of waste water disposal andenvironmental protection.

Comparative Example 6

The test piece was degreased with a solvent, treated with apyrophosphate salt, washed with water, post-treated with sodiumhydroxide, rinsed with water and then washed with pure water. Thus, thenumber of process steps was large and the amount of waste water waslarge. Thus, this method was inappropriate from the viewpoint of costs.

TABLE 1 Composition of treating liquids used in Examples and ComparativeExamples Tripoly- Pyro- Ortho- Defoam- Sodium phosphate phosphatephosphate Borate Surf- Chela- ing Example ion ion ion ion ion actantting agent No. (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) pH Ex. 13400 7500 0 0 100 0  0  0 9 Ex. 2 4600 10000 0 0 200 500 100 100 9 Ex. 34800 10000 0 0 1000 500 100 100 9 Ex. 4 20000 40000 0 0 1000 500 100 10011 Ex. 5 1000 2000 0 0 500 1000 100 100 8 Ex. 6 5600 0 10000 0 1000 1000100 100 9 C. Ex. 1 230 500 0 0 0 500 100 100 8 C. Ex. 2 2700 60000 0 0 0500 100 100 9 C. Ex. 3 7500 0 0 10000 0 500 100 100 10 C. Ex. 4 300010000 0 0 50 500 100 100 17 C. Ex. 5 Degreasing with solvent + pickling(HF solution) + Dow No. 7 method C. Ex. 6 Degreasing with solvent +pyrophosphoric acid treating liquid + post-treatment with sodiumhydroxide Surfactant: Polyoxyethylene nonylphenyl ether (EO: 11).Chelating agent: Sodium gluconate. Defoaming agent: Polyoxyethylenenonylphenyl ether (EO: 5).

TABLE 2 Deposited Amount of Initial Etched amount of residual adhesionSalt Example amount phosphorus carbon of coated spray No. (g/m²) (mg/m²)(mg/m²) film test Ex. 1 3.0 20 40 90 B Ex. 2 5.3 35 20 95 A-B Ex. 3 5.230 15 100 A Ex. 4 9.5 55 10 90 A-B Ex. 5 1.0 18 50 90 B Ex. 6 3.4 10 3090 B C. Ex. 1 0.3 2 150 40 C C. Ex. 2 13.0 55 10 50 C C. Ex. 3 0.1 0 18040 C C. Ex. 4 2.1 10 80 90 C C. Ex. 5 — — 10 100 A C. Ex. 6 — 5 15 90 BThe amount of residual carbon of a blank: 200 mg/m².

What is claimed is:
 1. A method for the surface preparation of anarticle of magnesium or its alloy comprising treating the article with atreating liquid which contains 1,000 to 20,000 ppm of alkali metal ions,1,000 to 50,000 ppm of condensed phosphate ions, and 100 to 20,000 ppmof borate ions, and has pH of at least
 8. 2. A method according to claim1, wherein said treating liquid further contains 10 to 2,000 ppm of asurfactant, 10 to 2,000 ppm of a chelating agent, and 10 to 2,000 ppm ofa defoaming agent.
 3. A method according to claim 1, wherein said alkaimetal ions are sodium ions.
 4. A method for coating an article ofmagnesium or its alloy comprising the steps of: treating the articlewith a treating liquid which contains 1,000 to 20,000 ppm of alkalinemetal ions, 1,000 to 50,000 ppm of condensed phosphate ions, and 100 to20,000 ppm of borate ions, and has pH of at least 8, removing thearticle from the treating liquid, washing the article with water, dryingthe-washed article, and coating the dried article.
 5. A method accordingto claim 4, wherein said dried article is coated with a powder coatingonce.